added method to destroy all textures for a given GL context
added a method to destroy a GL program wrapper and free associated resources
leave in a debug line, remove a stray line
add methods to destroy a single mode instances, or all modes for a given GL context
avoid race conditions that can lead to multiple copies of render mode GL programs
more thorough clean-up on scene destroy
expose layer in demo for debugging
update build
lint
mark scene when destroyed to prevent errors on further calls
leaflet layer: unhook leaflet events and destroy tangram scene when layer is removed
WIP leaflet tests (layer add works but known issues w/layer removal)
Merge branch 'master' into destroy

Conflicts:
	dist/tangram-worker.debug.js
	dist/tangram-worker.min.js
	dist/tangram.debug.js
	dist/tangram.min.js
leaflet layer remove test
stray console logs
don't upgrade browserify yet
Update leaflet_layer_spec.js

document potential test gotcha
ability to re-add the leaflet layer to a map after it was previously removed
added test case for removing the leaflet layer from a map and then re-adding it
Merge branch 'destroy'